JPWO2015182767A1 - Ball screw device - Google Patents

Abstract

The screw shaft of the ball screw device is a high carbon bearing steel or a steel material having an Ms point of 172 ° C. or less and a DI value of 2.8 or more, and (a) a ratio of an effective hardened layer, (b ) Metal structure of non-hardened layer, (c) Average amount of retained austenite, (d) Hydrogen amount on rolling surface, (e) Carbide area ratio at groove bottom, (f) Amount of retained austenite at 50 μm depth from raceway surface Is a specific range.

Description

  The present invention relates to a ball screw device, and more particularly to a ball screw device suitable for an injection molding machine.

  Ball screw devices are roughly classified into injection molding machine applications and conveyance / positioning applications. In recent years, there has been a demand for weight reduction of automobile parts, and as one means for that purpose, resin conversion of parts has been studied. Under such needs, the output required for injection molding machines has increased, and it has become necessary to increase the size of the device. In order to suppress this as much as possible, it is becoming increasingly important to extend the life of the ball screw device. Yes.

  The performance required for the ball screw device is that the movement in the axial direction is first performed stably. For that purpose, it is required that the shape of the screw groove does not change over a long period of time. As a cause of the change in the shape of the screw shaft, there is a dimensional change due to separation or wear in which the surface of the raceway surface is partially lost or due to expansion or contraction of the steel material. That is, it can be said that providing a screw shaft excellent in peeling resistance, wear resistance, and dimensional stability is a high-quality ball screw device.

  However, it is not easy to produce a product that satisfies all of these durability levels at a high level. In an injection molding machine, since a ball screw device is used as a thrust source, high stress is repeatedly applied to the ball screw device, so that it is necessary to improve the peel resistance. In the ball screw device, the nut moves in the axial direction with respect to the screw shaft, so foreign matter is likely to be mixed in, and the ball may be damaged when it passes through the circulating parts of the nut, which deteriorates the surface properties of the rolling element. Because it is easy, surface-origin type peeling called “peeling” is often observed. For this type of separation, optimization of the retained austenite on the raceway surface of the screw shaft is effective, and carburizing treatment is used for this purpose. However, retained austenite tends to change to martensite during long-term use, and causes a dimensional change due to expansion due to the density difference. Therefore, when carburizing treatment is performed in order to improve the peeling resistance, a reduction in dimensional stability must be allowed to some extent.

  On the other hand, when the focus is on dimensional stability, high-frequency heat treatment is used. High-frequency treatment can heat only the necessary part by generating heat on the steel surface by induction heating. As a method of manufacturing a screw shaft using high-frequency heat treatment, a round bar made of steel is grooved. There are a method of applying a hardened layer later and a method of applying a groove after applying a hardened layer to a round bar. In any method, the hardened layer causing the dimensional change is only on the surface of the screw shaft, and the same or better performance can be expected as compared with the carburizing treatment. However, it is not easy to improve the peel resistance by high-frequency heat treatment. In the high-frequency heat treatment, heat generation occurs on the surface, so overheating is likely to occur on the surface, and cracks due to overheating are likely to occur in steel materials with a large amount of carbon. Therefore, when performing high-frequency heat treatment, generally, medium carbon steel is used, and 0.5% C-based SAE4150 is actually used for the screw shaft. However, the amount of retained austenite effective for delamination depends on the carbon content in the steel, and a life extension effect can hardly be expected with a carbon content of about 0.5% by mass.

  For example, Patent Documents 1 and 2 can be referred to for the induction heat treatment in the manufacturing process of the screw shaft.

  Further, in a ball screw device for an injection molding machine, special peeling may occur when the usage environment becomes severe. This exfoliation is an exfoliation starting from a structural change called “white exfoliation” and is considered to be caused by hydrogen that has penetrated into the steel. It is considered that hydrogen is originally present in steel and that hydrogen is generated by decomposition of lubricating oil during use and enters the steel. When white peeling occurs, the effect of extending the life by increasing the amount of retained austenite is diminished.

Japanese Unexamined Patent Publication No. 2010-90924 Japanese Unexamined Patent Publication No. 2005-299720

  As described above, it is difficult to extend the life by improving the peel resistance and wear resistance as well as the dimensional stability, and white peeling due to hydrogen has not been studied so far. Accordingly, an object of the present invention is to provide a screw shaft that has superior dimensional stability, resistance to peeling including white peeling, and wear resistance.

  As a result of intensive studies by the inventors to solve the above problems, the following findings have been obtained. First, in order to improve the resistance to peeling other than white peeling, it is effective to control the amount of retained austenite, which can be achieved by increasing the carbon of the steel material. And, in order to ensure the dimensional stability while increasing the amount of retained austenite on the surface, as a result of detailed investigation of the mechanism of dimensional change, when martensite is the basic structure like carburized steel, the dimensional change is It is generated isotropically in both the radial direction and the axial direction of the shaft, but the core is based on a thermally stable ferrite phase, and it is hardly changed in the axial direction by ensuring a certain volume or more. I found it. The dimensional stability required for the ball screw device relates to the axial direction, and if this phenomenon is used, both life and dimensional stability can be achieved.

  Although it is not mainstream in ball screw devices for injection molding machines, when producing screw shafts by high-frequency heat treatment, groove processing may be performed after applying a hardened layer to a round bar. Since a certain degree of hardenability can be ensured by the conversion, a necessary hardened layer depth can be secured by adding a common amount of alloying elements, and there is a core region necessary for dimensional stability even at the hardened layer depth. It can be secured.

  However, the combination of the high carbon steel and the induction heat treatment has a problem of burning cracking and cracking during rebending. The former can be avoided by frequency conditions and strict temperature management. However, it is practically difficult to avoid the latter re-bending operation and cannot be avoided. When quenching is performed, the structure changes from austenite to martensite, and thermal deformation always occurs due to the density difference. Such a deformation can be reduced by grooving after applying a hardened layer as described above, but in the ball screw device for an injection molding machine, the grooving is performed in order to secure the necessary shape as a machine part. Since it is often subjected to high-frequency heat treatment later, deformation due to heat treatment appears as bending of the screw shaft, and it is necessary to correct this by bending. Since a groove is formed on the screw shaft, the groove bottom is greatly deformed during bending, and there is a great concern that cracks will occur if the allowable amount is exceeded. Therefore, as a result of detailed investigation of the influence of the structure on the bending strength, it was found that this can be avoided by setting the amount of carbide at the groove bottom to a certain level or more. This is because the crystal grains are kept fine due to the pinning effect of carbide, and the steel is high carbon steel, and the amount of carbon is as high as 1% by mass, so the amount of carbon that dissolves into the base decreases as the amount of carbide increases. This is due to the fact that toughness can be ensured.

  In addition, when we examined white flaking, by limiting the amount of hydrogen in the steel before use, it was possible to suppress the occurrence of white flaking, and only normal flaking would occur, making it virtually harmless. I found.

This invention is made | formed based on such knowledge, and provides the following ball screw apparatus.
(1) A screw shaft having a spiral groove on the outer peripheral surface, a ball nut having a spiral groove facing the spiral groove of the screw shaft on the inner peripheral surface, and interposed between the both spiral grooves and provided on the ball nut In a ball screw device comprising a plurality of balls that can be circulated by a ball circulation path,
The screw shaft is a high carbon bearing steel or a steel material having an Ms point calculated by the following formula 1 of 172 ° C. or less and a DI value calculated by the following formula 2 of 2.8 or more, and a thermosetting treatment; ,
The ratio of the effective hardened layer whose hardness in the radial cross section is HV500 or more is 60% or less,
The non-hardened layer having a hardness of less than HV500 is a metal structure containing a ferrite phase and a cementite phase,
The average retained austenite amount in the radial cross section is 4.5% or less,
The amount of hydrogen on the rolling surface of the spiral groove is 0.61 ppm or less,
The carbide area ratio of the groove bottom is 1.5% or more,
The amount of retained austenite at a depth of 50 μm from the raceway surface is 5 to 40% by volume.
A ball screw device.
Formula 1 = 550-361 [C] -39 [Mn] -20 [Cr] -17 [Ni] -5 [Mo]
Formula 2 = (0.2 [C] +0.14) × (0.64 [Si] +1) × (4.1 [Mn] +1) × (2.33 [Cr] +1) × (3.14 [ Mo] +1) × (0.52 [Ni] +1)
(In the formula, [C], [Si], [Mn], [Cr], [Mo], [Ni] are the contents of C, Si, Mn, Cr, Mo, Ni in the steel material (mass%). ).)
(2) The hardness (HRC) of the raceway surface of the screw shaft and the retained austenite amount (γR) at a depth of 50 μm from the raceway surface satisfy the following formulas (3) and (4): ) Ball screw device according to the description.
Formula 3: γR + 0.15 × HRC−19.1 ≧ 0
Formula 4: γR + 21.2 × HRC-1176 ≧ 0
(3) The ball screw device according to (1) or (2) above, wherein the prior austenite grain size at the groove bottom of the screw shaft is 30 μm or less.

  In the ball screw device of the present invention, a specific steel material is heat-cured on a screw shaft (a) a ratio of an effective hardened layer, (b) a metal structure of a non-hardened layer, (c) an average retained austenite amount, (d) Dimensional stability that is superior to that of the past by making the amount of hydrogen on the rolling surface, (e) the carbide area ratio at the groove bottom, and (f) the amount of retained austenite at a depth of 50 μm from the raceway surface, respectively. It has resistance to peeling including white peeling and wear resistance. Therefore, it is particularly useful for a ball screw device for an injection molding machine, and has high performance and long life.

It is a graph which shows the relationship between Ms point and life ratio. It is a graph which shows the relationship between DI value and lifetime ratio. It is a graph which shows the relationship between the amount of surface residual austenite, and a life ratio. It is a graph which shows the relationship between hardness and the amount of surface retained austenite. It is a graph which shows the relationship between the value of Formula 3, and a life ratio. It is a graph which shows the relationship between the value of Formula 4, and a life ratio. It is a graph which shows the relationship between an effective hardened layer ratio and the dimensional change rate of an axial direction. It is a graph which shows the relationship between the carbide area ratio of a groove bottom, and bending strength ratio. It is a graph which shows the relationship between the value of Formula 5, and bending strength ratio. It is a graph which shows the relationship between hydrogen amount and life ratio. It is a graph which shows the relationship between the value of Formula 6, and a life ratio.

  Hereinafter, the present invention will be described in detail with reference to the drawings.

  The ball screw device according to the present invention is interposed between a screw shaft having a spiral groove on the outer peripheral surface, a ball nut having a spiral groove facing the spiral groove of the screw shaft on the inner peripheral surface, and both the spiral grooves. It has a plurality of balls that can be circulated by a ball circulation path provided on the ball nut, and the screw shaft is made of a specific high carbon bearing steel, which is heat-treated to form an effective hardened layer on the surface. is there. There are no restrictions other than the screw shaft, such as balls and ball nuts.

That is, as a material for the screw shaft used in the present invention, a high carbon bearing steel or a steel material having an Ms point calculated by the following formula 1 of 173 ° C. or lower and a DI value calculated by the following formula 2 of 2.8 or higher. Is used.
Formula 1 = 550-361 [C] -39 [Mn] -20 [Cr] -17 [Ni] -5 [Mo]
Formula 2 = (0.2 [C] +0.14) × (0.64 [Si] +1) × (4.1 · 1 [Mn] +1) × (2.33 [Cr] +1) × (3.14 [ Mo] +1) × (0.52 [Ni] +1)
(In the formula, [C], [Si], [Mn], [Cr], [Mo], [Ni] are the contents of C, Si, Mn, Cr, Mo, Ni in the steel material (mass%). ).)

  There is no limitation as long as it is a high carbon bearing steel or a steel material that satisfies the formulas 1 and 2, but it is preferably a bearing steel containing 0.6% by mass or more of carbon and additionally containing chromium and manganese as essential components. . If the carbon content is less than 0.6% by mass, the heat treatment quality defined in the present invention cannot be obtained, so that the content is preferably 0.6% by mass or more. More preferably, the carbon content is 0.95% by mass or more. In addition, although there is no restriction | limiting in the upper limit of carbon content, when it exceeds 2 mass%, a coarse carbide | carbonized_material will remain | survive and it will affect lifetime resistance. High carbon bearing steel contains nearly 1% by mass of carbon, and even if about 0.5% by mass of carbon necessary for quenching is dissolved in the base, a sufficient amount of carbide can remain on the raceway surface. Abrasion resistance can be improved.

  Chromium is an element that improves hardenability and is preferably contained in an amount of 0.9% by mass or more. However, if the amount of chromium is excessive, the workability will decrease, so it is preferable to make it 2% by mass or less. Manganese is an element that improves hardenability as well as chromium. It is preferable to add 0.2% by mass or more. However, if manganese is excessive, the workability will be lowered, so it is preferable to make it 2% by mass or less.

  Moreover, it is preferable to contain molybdenum. As shown also in the Example mentioned later, wear resistance can be improved more by making the value of Formula 1 in which the content in the steel of molybdenum with chromium and manganese is related into a specific range. It is presumed that this is because chromium, manganese and molybdenum dissolve in the carbide and harden the carbide.

  Specifically, high carbon chromium steel of JSI G 4805, bearing steel of ISO 683-17, SUJ2-5 steel, 100CrMnSi6-4 steel, etc. are mentioned, SUJ2-5 steel and 100CrMnSi6-4 steel are preferred.

  And the round bar which consists of such high carbon bearing steel is heat-hardened, and the effective hardened layer of hardness Hv500 or more is formed in a surface layer part. As the thermosetting treatment, carburizing treatment may be used, but high-frequency heat treatment is preferable. In this high-frequency heat treatment, a round bar material made of the above steel material is inserted into a coil connected to a high-frequency power source, and a high-frequency current is passed through the coil. Thereby, an eddy current flows on the surface of the round bar material by the high frequency electromagnetic field, and the surface of the round bar material is heated. And a round bar raw material is heated over the full length by moving a coil to the axial direction of a round bar raw material. After heating, it is quenched by spraying an aqueous solution in which a water-soluble quenching solution is dissolved in a round bar material.

  The effective hardened layer has a volume ratio of 60% or less in the cross section of the screw shaft. That is, the processing conditions (frequency, output, heating time), steel composition, screw shaft diameter, etc. so that the depth from the surface on which the effective hardened layer is formed is 60% or less with respect to the screw shaft diameter. Adjust according to.

  An effective cured layer ratio of 60% or less means that the ratio of an uncured layer having a hardness of less than HV500 is 40% or more. The non-hardened layer is a barite containing a ferrite phase and a cementite phase, but since the ferrite phase has a sufficiently low carbon content, it hardly changes even during long-time use, and the hardened layer of the surface layer is in the core. As a result of restraining, even if the amount of retained austenite of the surface layer generated by the heat treatment increases, the dimensional change in the axial direction hardly occurs. In order to obtain such an effect, the ratio of the non-cured layer may be 40% or more, and 60% or more is more effective.

  The average remaining austenite amount in the radial cross section is 4.5% by volume or less. When the average retained austenite amount exceeds 4.5% by volume, the dimensional change rate increases.

  Furthermore, in the groove bottom, by making the carbide area ratio 1.5% or more and 5.6% or more, a particularly excellent bending strength is obtained in the bending test of the examples described later, and cracking in re-bending Effective for prevention. In order to prevent cracking, the prior austenite grain size at the groove bottom is preferably fine, specifically 30 μm or less, more preferably 18 μm or less.

  Further, the amount of retained austenite (surface retained austenite amount) at a depth of 50 μm from the raceway surface is set to 5 to 40% by volume. In the ball screw device, since the screw shaft is exposed, peeling due to foreign matter is likely to occur. However, in order to improve the life under foreign matter mixing while suppressing dimensional change, the surface retained austenite amount is 5 to 40 volumes. %, Preferably 9.7 to 35% by volume.

  Furthermore, in order to prevent white flaking, the amount of hydrogen on the rolling surface is made 0.61 ppm or less. The smaller the amount of hydrogen, the better, and 0.2 ppm or less is preferable.

In addition to the above, since a high contact surface pressure is applied between the screw shaft and the rolling element, the hardness (HRC) on the raceway surface is preferably as high as possible, but further between the surface retained austenite amount (γR). Therefore, it is preferable that the following expressions 3 and 4 are satisfied.
Formula 3: γR + 0.15 × HRC−19.1 ≧ 0
Formula 4: γR + 21.2 × HRC-1176 ≧ 0

  The present invention will be further described below with reference to examples, but the present invention is not limited thereto.

(Test 1)
A rod-shaped test piece made of a high carbon bearing steel having an alloy composition shown in Table 1 and having a diameter determined in consideration of a removal allowance by polishing was prepared, and transfer firing was performed by induction heat treatment at a frequency of 100 to 200 kHz. Next, a tempering treatment was performed at 160 to 200 ° C. for 2 hours, and 200 μm was removed from the black skin surface by polishing, and then subjected to a life test under the following conditions. In addition, in order to reproduce peeling of a hole screw apparatus, the rolling element used the ball | bowl which worsened the surface roughness beforehand as shown below. Moreover, Ms point was computed from the following formula 1. The results are shown in Table 2 and FIG. 1, and are shown as relative values (lifetime ratio) with respect to Comparative Example 2.
Formula 1 = 550-361 [C] -39 [Mn] -20 [Cr] -17 [Ni] -5 [Mo]
<Test conditions>
・ Test specimen: φ12.6 mm, rod-shaped specimen, rolling element: Material: SUJ2, Size: 1/2 inch, Surface roughness: 0.3 μmRa
・ Surface pressure ... 5.5 GPa
・ Lubrication ... Oil bath lubrication, VG68

  As shown in Table 2 and FIG. 1, it can be seen that the life extension effect is recognized in Examples 1 to 5, and the Ms point should be 173 ° C. or lower.

(Test 2)
As shown in Table 3, after using the rod-shaped test piece made of the steel material shown in Table 1, this rod-shaped test piece was subjected to transfer baking by induction heat treatment at a frequency of 30 to 100 kHz, and then tempered at 160 to 200 ° C. for 2 hours. After removing 4 mm from the surface of the black skin by polishing, it was subjected to a life test similar to Test 1. Further, the DI value was calculated from the following formula 2. The results are shown in Table 3 and FIG. 3, and are shown as relative values (lifetime ratio) with respect to Comparative Example 3.
Formula 2 = (0.2 [C] +0.14) × (0.64 [Si] +1) × (4.1 · 1 [Mn] +1) × (2.33 [Cr] +1) × (3.14 [ Mo] +1) × (0.52 [Ni] +1)

  As shown in Table 3 and FIG. 2, in Examples 6-10, the life extension effect is recognized and it turns out that DI value should just be 2.8 or more. In addition, the sixth embodiment and the eighth embodiment have substantially the same DI value, but the lifetime of the eighth embodiment is longer. This is considered to be due to the difference in quality between the two at the machining allowance position of 4 mm. That is, the steel material A (Ms point 155 ° C.) is used in Example 6, and the steel material C (Ms point 172 ° C.) is used in Example 8, and it is necessary to obtain a heat treatment quality of a certain level or more for the life extension effect. It can be seen that both the DI value and the Ms point are affected.

(Test 3)
As shown in Table 4, a rod-shaped test piece made of the steel material shown in Table 1 was used, subjected to induction heat treatment, and grooved to produce a screw shaft. And the hardness of the raceway surface of the produced screw shaft and the amount of surface retained austenite were measured. A ball screw device was produced using the produced screw shaft and subjected to the same life test as in Test 1. The results are shown in Table 4 and are shown as relative values (lifetime ratio) with respect to Comparative Example 6. The relationship between the life ratio and the amount of surface retained austenite is shown in a graph in FIG.

  In the steel material G used in Comparative Example 6, it is difficult to increase the amount of surface retained austenite by high-frequency heat treatment, and the life extension effect is not recognized. On the other hand, in the steel materials A, B, C, D, and E used in Examples 11 to 39, the surface retained austenite amount can be increased by increasing the output in the high-frequency heat treatment, and is increased to 9.7% by volume or more. By doing so, it is understood that a sufficient life extension effect can be obtained, and it is preferably 13% by volume or more.

  Further, the lifespan varies in the range where the surface retained austenite amount is 13% by volume or more. As a result of the examination, it was found that there is a correlation between surface retained austenite and hardness (HRC). FIG. 4 is a graph showing the relationship between the hardness and the amount of retained austenite for the comparative example and the example with a life ratio of 1.6 or more.

  Furthermore, it has been found that there is also a correlation between the life ratio and Formula 3 or Formula 4 expressed by surface retained austenite and hardness (HRC). The values of Equation 3 and Equation 4 are shown together in Table 4, and the relationship between the value of Equation 3 and the life ratio is graphed in FIG. 5, and the relationship between Equation 4 and the life ratio is graphed in FIG. As shown in the figure, the value of the expression 3 and the value of the expression 4 are 0 or more, and the life extension effect is obtained. Expressions 3 and 4 are converted into relational expressions related to surface retained austenite, and each is shown by a straight line in FIG. 4. The comparative example indicated by the plot “x” is surrounded by two straight lines related to Expression 3 or 4 It is outside the range of the specified area. That is, it is preferable to satisfy the expressions 3 and 4 simultaneously for extending the life.

(Test 4)
As shown in Table 5, the rod-shaped test pieces made of the steel materials shown in Table 1 were used, and screw shafts having different effective hardened layer ratios were produced by induction heat treatment. About the produced screw axis | shaft, the surface residual austenite amount (volume%) and the average residual austenite amount (volume%) were measured with the ratio (%) of the effective hardening layer. The surface residual austenite amount is the amount of retained austenite at a depth of 50 μm from the raceway surface, and was measured by X-ray after removing the surface layer by 50 μm from the raceway surface. Further, the core portion (region having a hardness of less than Hv500) was chemically analyzed to obtain a metal structure. Further, after the induction heat treatment, before and after tempering again at a low temperature for a predetermined time, the dimensional change rate in the axial direction was measured, and a relative value (dimensional change rate ratio) with respect to Comparative Example 9 was obtained. Each result is shown in Table 5, and FIG. 7 is a graph showing the relationship between the effective hardened layer ratio and the dimensional change rate in the axial direction.

  In Examples 38 to 45, the axial dimension hardly changes even when the proportion of the effective cured layer increases. On the other hand, in Comparative Examples 7 to 9, the axial dimension increases as the proportion of the effective hardened layer increases. In each of the examples, the proportion of the effective cured layer is 60% or less, and it can be seen that the dimensional stability is improved by making the proportion of the effective cured layer 60% or less.

  Moreover, although the comparative example 9 imitates the screw axis | shaft of the present ball screw apparatus, it is a dimensional change amount equivalent to Examples 38-45, although the surface residual austenite amount is small. . From this, it can be said that making the average retained austenite amount 4.5% by volume or less is also effective for dimensional stability.

  Further, in Examples 38 to 45, the surface retained austenite amount is 5 to 40% by volume, and the dimensional change rate is small. From this, it can be said that it is also effective to set the surface retained austenite amount to 5 to 40% by volume.

  In addition, as a result of the chemical analysis of the core part, except for Comparative Example 9, all were pearlite composed of a ferrite phase and a cementite phase.

(Test 5)
As shown in Table 6, a rod-shaped material (diameter: 12.8 mm) made of a steel material shown in Table 1 was used, and a 1.5 R groove was formed on the circumference at the center in the longitudinal direction. The groove depth is 1.5 mm and the groove width is 3 mm. Then, the screw shaft was produced by high-frequency heat treatment at a frequency of 10 to 30 kHz. For the produced screw shaft, the carbide area ratio at the groove bottom and the crystal grain size of prior austenite were measured. The results are also shown in Table 6.

  Further, the produced screw shaft was subjected to a four-point bending test, and the bending strength was measured. Although a result is written together in Table 6, it shows as a relative value (bending strength ratio) with respect to the comparative example 10 which imitated the present product. FIG. 8 is a graph showing the relationship between the carbide area ratio at the groove bottom and the bending hardness ratio. In any screw shaft, it was confirmed that a crack occurred from the bottom of the groove, and at the same time, the crack did not stop and led to the fracture of the screw shaft.

  Comparative Example 10 uses the current steel material and simulates the same quality as the current one, but Comparative Example 11 has a lower bending strength than Comparative Example 10. In Comparative Example 11, the set output is increased in the high-frequency heat treatment to increase the amount of carbon penetration, and this is because the deformability of the matrix is reduced.

  In Examples 46 to 56, the set output is set lower than that of Comparative Example 11, and the bending strength is equal to or higher than that of the current Comparative Example 10. The bending hardness generally tends to increase as the hardness decreases. However, when high carbon steel is used as in the present invention, residual austenite which is a soft structure when the amount of carbon penetration increases as in Comparative Example 11 Since the amount increases and as a result the hardness decreases, it is not suitable as an evaluation index. A reasonable evaluation index is the solid solution amount of carbon in martensite, but it is not easy to quantitatively measure the solid solution amount. In the present invention, since bearing steel is used, the amount of solute carbon can be determined by subtracting the amount of carbon not dissolved in the material, that is, the amount of residual carbides, from the amount of carbon in the material. The bending strength is improved when the amount of residual carbide is large, that is, the amount of dissolved carbon is small. From Examples 46 to 56, when the area ratio of carbide at the groove bottom is 1.5% or more, the bending strength is equal to or higher than the current level. It can be seen that strength is obtained.

  Further, when the correlation between the bending strength and the prior austenite grain size was examined, it can be said that the grain size of the prior austenite at the groove bottom is preferably 30 μm or less from the comparison between Comparative Example 11 and Examples 46 to 56. .

Furthermore, from these results, it was found that there is a correlation between the value of the following formula 5 regarding the carbide area ratio of the groove bottom and the prior austenite grain size and the bending strength ratio. The value of Formula 5 is shown together in Table 6, and the relationship with the bending strength ratio is graphed and shown in FIG. 9. When the value of Formula 5 is 22 or more, preferably 35 or more, the bending strength ratio is Can be increased.
Formula 5 = Carbide bottom area ratio of groove bottom + 50−old austenite grain size

(Test 6)
Based on the above test results, a ball screw device BS6316-10.5 (nominal: JIS B1192 63 × 16 × 300-Ct7) was prepared and mounted on a ball screw durability tester manufactured by NSK Ltd. for durability test. Went. The test conditions are as follows. Further, in Examples 57 to 61, high-frequency heat treatment was performed, and in Example 62 and Comparative Example 12, carburizing treatment was performed to adjust the amount of enriched gas, thereby adjusting the amount of hydrogen on the raceway surface. The amount of hydrogen is determined by cutting the raceway surface of the screw shaft before the test and forming a solid whose upper surface of a 10 mm square cube is the raceway surface, and then heating from room temperature to 400 ° C. to release the total amount of hydrogen released. This is a value measured by an analyzer. The life is indicated by a relative value (life ratio) to the calculated life of a ball screw device using a screw shaft made of steel G. The results are shown in Table 7. Table 7 also shows the DI value, Ms point, and surface retained austenite amount of the produced screw shaft together with this hydrogen amount. FIG. 10 is a graph showing the relationship between the amount of hydrogen and the life ratio.
<Test conditions>
-Screw shaft outer diameter: 63 mm
・ Lead: 16mm
-Ball diameter: 12.7 mm
・ Test load: 300kN
・ Maximum rotation speed: 3200 min ⁻¹ (Dn: 200,000)
・ Nut material: SCM420
・ Material of separator: 66 nylon ・ Circulation method: return tube method ・ Lubricant: “YS2 Grease” manufactured by Lube Co., Ltd.

  As shown in Table 7, in Examples 57 to 62, the Ms point is 173 ° C. or less, the DI value is 2.8 or more, the amount of surface retained austenite is 5 to 40% by volume, and the hydrogen amount on the raceway surface is further increased. It is 0.61 ppm or less, and all of them have a lifetime that is twice or more the calculated lifetime.

Further, the present inventors have found that there is a correlation between the value of Formula 6 expressed by the amount of hydrogen on the raceway surface and the surface retained austenite and the life ratio. Table 7 shows the value of Equation 6 together, and FIG. 11 is a graph showing the relationship between the value of Equation 6 and the life ratio. If the value of Equation 6 is 40.2 or less, the life ratio is greatly extended. be able to.
Formula 6 = (hydrogen amount of raceway surface) × 15 + surface retained austenite amount

Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention.
This application is a Japanese patent application filed on May 30, 2014 (Japanese Patent Application No. 2014-112386), a Japanese patent application filed on June 10, 2014 (Japanese Patent Application No. 2014-119969), and an application filed on November 4, 2014. Based on Japanese Patent Application (Japanese Patent Application No. 2014-2224033), Japanese Patent Application (Japanese Patent Application No. 2015-016610) filed on January 26, 2015, Japanese Patent Application (Japanese Patent Application No. 2015-013626) filed on January 27, 2015 The contents of which are incorporated herein by reference.

  Since the ball screw device of the present invention has superior dimensional stability, resistance to peeling including white peeling, and wear resistance, it is particularly useful for ball screw devices for injection molding machines. Long life with performance.

Claims (3)

A screw shaft having a spiral groove on the outer peripheral surface, a ball nut having a spiral groove facing the spiral groove of the screw shaft on the inner peripheral surface, and a ball circulation path interposed between the spiral grooves and provided in the ball nut In a ball screw device comprising a plurality of balls circulated by
The screw shaft is a high carbon bearing steel or a steel material having an Ms point calculated by the following formula 1 of 172 ° C. or less and a DI value calculated by the following formula 2 of 2.8 or more, and a thermosetting treatment; ,
The ratio of the effective hardened layer whose hardness in the radial cross section is HV500 or more is 60% or less,
The non-hardened layer having a hardness of less than HV500 is a metal structure containing a ferrite phase and a cementite phase,
The average retained austenite amount in the radial cross section is 4.5% or less,
The amount of hydrogen on the rolling surface of the spiral groove is 0.61 ppm or less,
The carbide area ratio of the groove bottom is 1.5% or more,
The amount of retained austenite at a depth of 50 μm from the raceway surface is 5 to 40% by volume.
A ball screw device.
Formula 1 = 550-361 [C] -39 [Mn] -20 [Cr] -17 [Ni] -5 [Mo]
Formula 2 = (0.2 [C] +0.14) × (0.64 [Si] +1) × (4.1 [Mn] +1) × (2.33 [Cr] +1) × (3.14 [ Mo] +1) × (0.52 [Ni] +1)
(In the formula, [C], [Si], [Mn], [Cr], [Mo], [Ni] are the contents of C, Si, Mn, Cr, Mo, Ni in the steel material (mass%). ).) 2. The ball according to claim 1, wherein a hardness (HRC) of a raceway surface of the screw shaft and a retained austenite amount (γR) at a depth of 50 μm from the raceway surface satisfy the following expressions 3 and 4. Screw device.
Formula 3: γR + 0.15 × HRC−19.1 ≧ 0
Formula 4: γR + 21.2 × HRC-1176 ≧ 0   3. The ball screw device according to claim 1, wherein a prior austenite grain size at a groove bottom of the screw shaft is 30 [mu] m or less.

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